The present invention relates to methods and systems for detecting leaks by aspirating gases at an aspiration inlet and detecting a tracer gas, such as helium, by means of a detector, such as a mass spectrometer, associated with primary and secondary pumps.
In prior art methods and systems of the above kind, a mass spectrometer is connected to the aspiration side of a secondary pump whose discharge side is connected to the aspiration side of a primary pump driven by a rotary drive system. The gases are aspirated via a detector input that can be selectively connected by control valves either to the aspiration side of the secondary pump or to the discharge side or an intermediate area of the secondary pump.
In a leak detector of the above kind using a mass spectrometer, there are three basic test modes:
a. the total contraflow test, in which the aspiration input of the detector is connected to the discharge side of the secondary pump and thus to the aspiration side of the primary pump: the mass spectrometer then detects the contraflow of a fraction of a tracer gas, such as helium, in the secondary pump;
b. the direct test, in which the aspiration inlet of the detector is connected to the aspiration side of the secondary pump, i.e. to the input of the mass spectrometer: the mass spectrometer then detects directly the tracer gas, such as helium, coming from the input of the detector;
c. the semi-direct or partial contraflow test, in which the aspiration inlet of the detector is connected to an intermediate area between the aspiration side and the discharge side of the secondary pump: the mass spectrometer then detects the contraflow of the tracer gas, such as helium, in the portion of the secondary pump between the connection area of the aspiration inlet of the detector and the aspiration side of the secondary pump.
In practice, during a first pre-evacuation stage, the inlet of the detector is generally connected to the aspiration side of the primary pump, which aspirates the gases until an intermediate pressure threshold enabling the total contraflow test is reached. The intermediate threshold depends on the secondary pump, but is generally in the order of 10xe2x88x921 hPa to 10 hPa.
When the pressure threshold is reached on the aspiration side of the primary pump, the secondary pump can operate correctly. Its discharge side is then connected to the aspiration side of the primary pump by opening a connecting valve and a total contraflow test step can be carried out.
Subsequently, when the pressure at the inlet of the detector has reached a lower pressure threshold which is sufficiently low, generally less than 10xe2x88x922 hPa, the direct test mode can be selected by connecting the inlet of the detector directly to the aspiration side of the secondary pump and disconnecting its connection to the aspiration side of the primary pump.
Clearly the characteristics of the primary pump used on the leak detector directly affect the performance of the detector. This is because the pumping speed of the primary pump determines the duration of the pre-evacuation phase or the waiting time before the test can begin. The pumping speed of the primary pump also determines the maximum flow which can be admitted to the detector for the mass spectrometer to be able to detect the tracer gas effectively in a total contraflow test. Equally, the aspiration speed of the primary pump determines the sensitivity and the response time of the detector when it is used in the total contraflow test mode. Finally, the primary pump generally has a limited capacity to evacuate the tracer gas downstream of the secondary pump, and this produces a spurious signal from the mass spectrometer, resulting from the contraflow of tracer gas through the secondary pump in the direct or semi-direct test mode.
A first problem addressed by the present invention is that of reducing the pre-evacuation time in order to begin the test as quickly as possible with the same primary pump.
Another problem addressed by the invention is that of enabling a leak detector to detect larger leaks or to compensate for a high level of outgassing with the same primary pump.
Another problem addressed by the invention is that of adapting the sensitivity of the leak detector to the characteristics of the leak to be detected, using the same primary pump.
Another problem addressed by the present invention is that of eliminating helium background noise from the primary pump by eliminating the spurious signal resulting from the contraflow of tracer gas, such as helium, in the secondary pump during a direct mode test.
To achieve the above and other objects, the invention provides a method of detecting leaks by aspirating gases at an aspiration inlet and by detecting a tracer gas by means of a detector, such as a mass spectrometer, associated with a primary pump and a secondary pump, the method comprising at least a first aspiration step during which the primary pump is driven in rotation in a first speed range and a second aspiration step during which the primary pump is driven in rotation at a second speed outside the first speed range.
In a first embodiment of the invention, the second speed is the nominal speed of the primary pump and the speeds of the first speed range are greater than the nominal speed of the primary pump.
In a first application of the invention, the first speed range is used during a limited first pre-evacuation step period during which the aspiration inlet is connected directly to the aspiration side of the primary pump.
In a second application of the invention, the first speed range is used during a total contraflow test step to compensate for a high level of outgassing or to detect grosser leaks.
In a second embodiment of the invention, the speeds of the first speed range are less than the nominal speed of the primary pump and the first speed range is used during a total contraflow test step to reduce the pumping speed and thereby increase the sensitivity of detection.
In a third embodiment of the invention, the first and second speeds being different, the speeds and the signals produced by the detector, such as the mass spectrometer, during each of the two aspiration steps are stored, and the stored values are used to compute the usable signal resulting from the tracer gas coming from the aspiration inlet by subtracting the spurious signal resulting from the contraflow of tracer gas through the secondary pump.
In a particularly advantageous application of the third embodiment of the invention:
the rotation speed of the primary pump is partially modulated with a low-frequency alternating speed component,
the modulated component is dissociated in the total signal coming from the mass spectrometer, and
the spurious signal associated with the rotation speed of the primary pump is computed from the modulated component and the degree of modulation of the rotation speed of the primary pump and subtracted from the total signal to deduce the usable signal therefrom in real time.
A leak detector system in accordance with the invention for implementing the above method comprises a mass spectrometer connected to the aspiration side of a primary pump driven by a rotational drive system, with an aspiration inlet selectively connectable by control valves either to the aspiration side of the primary pump or to the aspiration side or an intermediate area of the secondary pump; the rotational drive system of the primary pump is adapted to vary the rotation speed of the primary pump between at least two different rotation speeds.
A rotational drive system can advantageously be provided that is adapted to modulate the rotation speed of the primary pump with a low-frequency alternating speed component.
In this case, a speed sensor can advantageously be provided for measuring the rotation speed of the primary pump, means for storing the time sequence of signals from said speed sensor and signals produced by the mass spectrometer, and computation means for computing the usable signal from the stored signals.